Permanent magnet motors and generators are subject to the tendency to permanently demagnetize the permanent magnet field poles as a result of armature reaction. To prevent or minimize such demagnetization, various arrangements of field coils have been proposed heretofore which are in circuit with the armature to provide a cumulative MMF to counteract the demagnetization. One known field compensation technique employs a split series aiding field to cause armature currents to pass through the field with sufficient turns in the field to protect the permanent magnet poles from the demagnetization. Such split series aiding fields are shown, for example, in U.S. Pat. Nos. 3,427,484 and 3,566,251 and German Pat. Nos. 711,343 and 506,984.
For applications in which the motor is part of a bidirectional servocontrol system, the field is usually excited by a switching type drive amplifier such as an SCR amplifier, which provides positive and negative drive signals in accordance with the intended direction of motor rotation. The driving signals may be rapidly switched to provide forward and backward rotation during servo operation and simultaneous application of the positive and negative driving signals can occur which can cause destructive short circuit currents. The driving signals are usually applied to the motor by way of a center tapped inductor which serves to limit the short circuit current in the event of cross firing of the drive amplifier.